1. Field of the Invention
This invention relates to electrical switching apparatus such as circuit interrupters and contactors having a digital trip unit, and more particularly, to such apparatus usable with both 50 Hz and 60 Hz power systems.
2. Background Information
Circuit breakers are widely used in industrial, commercial and residential applications for protecting electrical conductors and apparatus from damage due to excessive current flow. Initially used as direct replacement for fuses, circuit breakers have been gradually called upon to provide more sophisticated types of protection other than merely interrupting the circuit when the current flow exceeds a certain level. More elaborate time-current trip characteristics have been developed such that a circuit breaker can rapidly open upon very high overload conditions, but delays interruption upon detection of lower overload currents with the time delay being roughly inversely proportional to the degree of overload. Circuit breakers are also available which interrupt upon the detection of ground fault currents. As the complexity of electrical distribution circuits has increased, the control portions of the circuit breaker have been interconnected to provide selective coordination.
During the late 1960's, solid state electronic control circuits were developed for use in high power, low voltage circuit breakers. These electronic control circuits performed functions such as instantaneous and delayed tripping which were traditionally achieved by magnetic-thermal means. The improved accuracy and flexibility of the solid state electronic controls resulted in their wide spread acceptance.
The earliest electronic control circuit designs utilized discrete components such as transistors, resistors and capacitors. More recent designs such as that disclosed in U.S. Pat. No. 4,428,022 have included microprocessors which provide improved performance and flexibility. Due to the severe space limitations in the low voltage circuit breakers, the assignee of this application has developed a special purpose integrated circuit known as a SuRE Chip which incorporates a microcontroller core processor, volatile and nonvolatile memory, and an 8-bit analog to digital converter with a six-input multiplexer which provides all of the essential analog and digital functions in a single monolithic device. This device is described in detail in copending U.S. patent application Ser. No. 07/636,643 filed on Dec. 28, 1990.
These digital systems sample the current waveforms periodically to generate a digital representation of the current waveform. In accordance with the well known Nyquist criteria, a sinusoidal waveform must be sampled at a sampling rate which is greater than twice the frequency to be detected. Higher sampling rates provide the capability of earlier detection of transients such as short circuits. U.S. Pat. No. 5,060,166 suggests sampling the analog currents every 90 electrical degrees, or four times per cycle, to detect changes in amplitude within less than one half cycle. This technique presupposes clean sinusoidal current waveforms.
Recent increase in the use of power conditioning equipment and other non-linear loads has resulted in an increase in the harmonic content of the current waveforms seen by the circuit breaker. A paper entitled "RMS DIGITAL TRIPS OFFER INCREASED ACCURACY AND RELIABILITY ADVANCES IN LOW VOLTAGE CIRCUIT BREAKER TRIP TECHNOLOGY" by Purkajastha et al., 35th Petroleum and Chemical Industry Conference, Dallas, Tex., September 12-14, pages 157-163, addresses this problem and finds that the prior art peak detecting trip units calibrated in rms assuming steady state sinusoidal current can generate nuisance trips due to the harmonics. This paper recommends that harmonics up to the thirteenth be detected and suggests that the analog currents be digitized at a rate of 27 samples per cycle. This sampling rate satisfies the Nyquist criteria of sampling at better than twice the frequency to be detected. However, this high sampling rate places a burden on the microprocessor which, due to space limitations in the circuit breaker casing, has limited processing capability.
With the increasing globalization of markets, circuit breakers must be compatible with the local power systems in different parts of the world. Thus, they must be adaptable for operating at a fundamental frequency of 50 Hz and 60 Hz.
While a sampling rate meeting the Nyquist criteria for 60 Hz power will be adequate for 50 Hz, a single sampling rate synchronized to one frequency will not be synchronized to the other. As will be seen, the sampling rate of the described circuit breaker or contactor must be synchronized to the ac power, whichever the frequency of the source.
The long delay trip function of circuit breakers, and overload protection in contactors, model heating of the load, and interrupt current when predetermined limits are reached. The cooling of the load following interruption of load current is then mimiced so that the load cannot be reenergized until it safe to do so. Circuit breakers and contactors are typically powered by the currents which they interrupt. In the electromechanical analog devices where a bimetal is used to model the heat state of the load, the cooling model is not dependent on power. However, the interruption of load current interrupts the normal operation of microprocessor based circuit breakers and hence their ability to track cooling of the unpowered load. U.S. Pat. No. 5,136,458 teaches forcing the voltage on an external capacitor to track the I.sup.2 t load heating characteristic modeled by the microprocessor. When current to the load, and the microprocessor, is interrupted, the capacitor discharges through a resistor selected to bleed the voltage off of the capacitor at a rate which mimics cooling of the load. Another approach has been to charge a memory capacitor when a trip occurs. The voltage on this capacitor is bled and read on power-up so that the microprocessor knows that the trip occurred and shortens the next trip time based on the time since the last trip.
The rate of discharge of these memory capacitors is necessarily very slow. It has been found that this can be a nuisance when calibrating or field testing the circuit breaker or contactor. There is a need for a means for rapidly discharging these memory capacitors for calibration and test purposes, but which is not easily accessible to a user who might try to defeat the normal operation of the protective device and repower the load before it is safe to do so.